1. Field of the Invention
The present invention relates to a single crystal pulling apparatus used for growing a single crystal by a CZ method.
2. Description of the Related Art
The CZ method is often used as a method of growing a silicon single crystal used as a material for a semiconductor device. In growing the silicon single crystal by the CZ method, as shown in FIG. 3, a seed crystal 3 is immersed in a silicon melt 2 formed in a crucible 1, and from this condition, the seed crystal 3 is pulled upward while rotating the crucible 1 and seed crystal 3 to thereby grow a silicon single crystal 4 beneath the seed crystal 3.
The crucible 1 has a double structure of an inner quartz crucible 1a and an outer graphite crucible 1b. The double crucible is placed on a crucible tray 5, and moved up and down and rotated in accordance with up and down movement and rotation of a pedestal 6 supporting the crucible tray 5. The outer graphite crucible 1b is a holding crucible for reinforcing mechanical strength of the inner quartz crucible 1a. There are two types of graphite crucibles: a one body type and a divided type.
In growing the silicon single crystal by the CZ method, the size and weight of the crystal have been rapidly increased in recent years, causing crucibles of increased size to be used. When a large one body type graphite crucible is used, handling is not easy to thereby increase burden on an operator, and when a quartz crucible is recovered from the graphite crucible after pulling, the quartz crucible firmly adheres to an inner surface of the graphite crucible due to thermal expansion difference between quartz and graphite to cause problems of difficulties in recovery of the graphite crucible. Thus, for a large crucible, a divided type graphite crucible is generally used that is easier to handle and also offers easier recovery of the quartz crucible than the one body type graphite crucible.
As shown in FIG. 4, a divided type graphite crucible 1b is circumferentially divided into a plurality of parts by radial dividing lines 1d, and generally has a three way divided structure of combined three divided pieces 1c. In the divided type graphite crucible, crucible fastening means for fastening the divided pieces to be combined is used as required, and one of such fastening means is described in Japanese Patent Laid Open No. 2000-264777 specification. The described crucible fastening means has a lid-like or cap-like retainer fitted into an opening provided in a center of its bottom to thereby prevent outward fall of a plurality of divided pieces formed by circumferentially dividing the crucible, and to avoid enlarging a gap between adjacent divided pieces.
In growing the silicon single crystal by the CZ method, inner quartz crucibles have been disposable and not been reused. This is because physical and chemical damage in use is significant. However, recent studies have revealed that the quartz crucibles suffer fatal damage from use simply on their inner surface layers, and that removing the surface layers allows reuse by relatively easy recycling process. From this point of view, attempts have been made to recycle and reuse the quartz crucibles, and in such a case, using the divided type graphite crucibles is essential for recovering the quartz crucibles from the graphite crucibles.
For the divided type graphite crucible assuming that the quartz crucible is recovered and reused, it is not preferable to firmly fasten the divided pieces since it causes adhesion of the quartz crucible and difficulty in recovery. That is, the divided pieces need to have flexibility of such a degree that the divided pieces may have a slight gap therebetween. Therefore, specific fastening means is not used for firmly fastening the divided pieces, and a conventional divided structure is used where the graphite crucible is simply circumferentially divided into a plurality of parts as shown in FIG. 4. Such a simple divided type graphite crucible has flexibility of following deformation of the quartz crucible and facilitates recovery of the quartz crucible after use, while displacement of the graphite crucible itself due to the deformation of the quartz crucible may damage the quartz crucible.
That is, the quartz crucible is mainly deformed to have an enlarged opening. When the opening of the quartz crucible is enlarged, the divided pieces of the quartz crucible tilt outward like a nail puller. Specifically, each divided piece has a vertical portion formed by circumferentially dividing a straight body portion of the crucible, and a horizontal portion formed by circumferentially dividing a bottom of the crucible. When the opening of the quartz crucible is enlarged, the vertical portion falls outward to cause the horizontal portion to tilt upward pivoted on its periphery. The horizontal portion has a sharp triangular tip at its center, so that tilting due to the fall of the vertical portion causes the tip to locally push up a center of the bottom of the quartz crucible and to thereby easily damage the crucible in a softened state.
The fastening means described in Japanese Patent Laid-Open No. 2000-264777 specification completely fastens the divided pieces to eliminate their flexibility, which tends to cause adhesion of the quartz crucible. Thus, it is inappropriate as the fastening means in the divided type graphite crucible assuming that the quartz crucible is recovered and reused.
An object of the present invention is to provide a single crystal pulling apparatus capable of imparting flexibility suitable for recovery of a quartz crucible to a graphite crucible and also capable of effectively avoiding damage of the quartz crucible due to the flexibility.
In order to attain the above described object, a single crystal pulling apparatus according to the present invention includes: a quartz crucible containing a melt; a graphite crucible holding the quartz crucible, provided with an opening in a center of its bottom, and divided into two or more parts by radial dividing lines around the opening; and a crucible tray on which the graphite crucible is placed and that has a protrusion fitting into the opening on a crucible placing surface.
In the single crystal pulling apparatus according to the present invention, a plurality of divided pieces constituting the graphite crucible includes a vertical portion formed by circumferentially dividing a cylindrical straight body portion and a horizontal portion formed by circumferentially dividing a circular bottom around the opening. The protrusion fitting into the opening is provided on the crucible placing surface of the crucible tray to thereby support the quartz crucible and not to hinder displacement of the divided pieces caused by an enlarged opening of the quartz crucible. When the quartz crucible is deformed, the vertical portion of the divided piece falls outward to cause the horizontal portion to tilt upward pivoted on its periphery. However, the horizontal portion has a fan shape with an arcuate periphery and center, so that there is a low risk of damage to the bottom or the quartz crucible even when it tilts upward pivoted on its periphery.
More preferably, as a mechanism for positioning the divided pieces of the graphite crucible, an annular projection is provided on a lower surface of the bottom of the graphite crucible so as to be positioned on a periphery of the opening, and an annular recess into which the annular protrusion fits is provided on the crucible placing surface of the crucible tray. The positioning mechanism radially fastens the plurality of divided pieces constituting the graphite crucible, but allows fall and tilt caused by the enlarged opening of the quartz crucible, and does not hinder flexibility required for separation and recovery of the quartz crucible.
An inner diameter of the opening is preferably 0.04 to 0.9 times an inner diameter of the straight body portion of the graphite crucible. An inner diameter less than 0.04 times thereof causes a central end of the divided piece to tend to be sharpened and may cause damage to the bottom of the quartz crucible. Also, a protrusion of the crucible tray is thin to thereby increase the risk of breakage. On the other hand, an inner diameter more than 0.9 times thereof significantly reduces a radial dimension of the horizontal portion of the divided piece to reduce stability of the divided piece, thereby increasing the risk of a drop from the crucible tray. The most preferable inner diameter of the opening is 0.07 to 0.50 times the inner diameter of the straight body portion of the graphite crucible.
An outer diameter of the crucible tray is preferably 0.5 or more times an outer diameter of the straight body portion of the graphite crucible. An outer diameter less than 0.5 times thereof may reduce a function of radially fastening the divided pieces to cause a gap between the divided pieces to be excessively enlarged.
Minimum thickness of the crucible tray (generally, thickness of a portion provided with the annular recess) is preferably 15 mm or more. Minimum thickness less than that may cause the periphery of the crucible tray to be bent downward and cause the gap between the divided pieces to be excessively enlarged.
The present invention is especially suitable for pulling by a three or more way divided graphite crucible in which a tip of the divided piece tends to be sharpened.